1. Field of the Invention
The present invention relates to an electric pump having a linear motor, and more particularly to such an electric pump intended to be installed at the bottom of a well, for example an oilwell.
2. Description of Related Art
In some oilwells, the natural flow of hydrocarbons from the bottom to the surface proves to be insufficient to allow or maintain commercial production. This is due either to the high viscosity of the hydrocarbons, or too low a natural pressure at the bottom of the well, or else to a combination of the two. In order to enable the well to be brought into production on a commercial scale, a well-assistance system or well-stimulation system may be used. For example, it is possible to place a pump on the lower end of production tubing located in the well.
Rod pumping units have already been proposed, which consist of a downhole positive-displacement pump fitted in the tubing, the piston of which is driven in translational motion from the surface by means of steel or glass-fibre rods. On the surface, the motion is imparted to the string of rods by a structure having a mule head driven by a rotary electric motor or else a hydraulic power cylinder.
The deadweight, inertia, friction and mechanical fatigue of the rods limit the pumping capacity and performance of these systems. They are ill-suited to blowing wells on which downhole safety devices are required, to deep wells or to high output levels (greater than 200 m.sup.3 /d of liquid).
Alternatively, the pump may be driven by a submerged electric motor at the bottom of the well, which is supplied via a cable placed in the annular space between the tubing and the casing of the well.
An example of this type of pump is given in document U.S. Pat. No. 4,928,771 which describes a rotary pump driven by an electric motor placed in the well above the pump. This type of pump has drawbacks, firstly because it is bulky, the pump and the motor forming two separate units, and, in addition, because the two units are submerged in the fluid flowing in the well. This fluid constitutes an aggressive medium which is the cause of a large number of breakdowns experienced by this type of pump.
Electric pumps driven by linear motors have also been proposed. In such pumps, a linear electric motor sets in motion the piston of a reciprocating pump. Document U.S. Pat. No. 4,687,054 describes an electric pump having a linear motor, intended to be placed at the bottom of an oilwell, the linear motor being placed above the pump which forms a separate subassembly. The fact that the motor and the pump form two separate subassemblies makes the electric pump bulky and heavy. The operations of fitting the electric pump into the well, operations which are effected by cable or by means of a small-diameter tube, and its periodic removal for maintenance, are made more difficult and laborious by the presence of the two subassemblies and by their weight. In addition, an electric pump formed by two subassemblies has a high inertia and, moreover, the connection between the sub-assemblies constitutes a weak point in the electric pump.